This July, more than 700 zombies descended on Lakewood, Colorado, sending thousands of local residents scurrying. This was no movie set. It was an event hosted by Reed Street Productions, a Baltimore-based company that puts on zombie-themed 5K races, dubbed Run for Your Lives.

Co-founders Ryan Hogan and Derrick Smith started the races last year as a way to sell inventory from Warwear, Hogan's athletic-apparel company. They added zombies to the mix, inspired by the popularity of AMC's The Walking Dead. It appears to be a winning combo: The company is on track to hit $15 million in sales this year.

Runners pay about $80 to race through an obstacle course that includes mudslides, climbing walls, and, of course, hordes of the undead. It's $25 to be a zombie--Reed Street provides makeup and costumes. The zombie slots usually fill up first.

"One of our biggest questions has been, 'Holy crap! How did we sell out of zombies so quick?'  " says Hogan. He plans to raise the price to $35 next year.

Reed Street is organizing zombie races in 13 cities this year and expects to double that in 2013. This month, the company is hosting a two-day event in Southern California that Hogan expects will draw up to 25,000 participants.

"I think people are fascinated with the concept," says Hogan. "Or they just love zombies."